1. Field of the Invention
The present invention relates to camera exposure control and flash control systems and more particularly to the combination of electronic flash tube control circuitry and exposure time control circuitry in cameras having a slit type shutter mechanism wherein a slit scans or sweeps over a film for photographic exposure. Although the invention is not restricted to the following, one of many typical examples of such a slit type shutter mechanism is a two-curtain type focal plane shutter having a first or leading shutter curtain and a second or trailing shutter curtain which travel one after another at a manually or automatically determined interval. In the case of high shutter speed with such a focal plane shutter, the second shutter curtain starts traveling to cover the film before the first shutter curtain completes its travel to fully uncover the film, whereby the film is exposed with the slit formed between the first and second shutter curtains "sweeping" the film surface. The present invention contemplates providing a combined camera and flash system effective for use with slit type shutter mechanisms that function in accordance with the above mode of operation.
2. Description of the Prior Art
It is well known that an ordinary electronic flash tube emits flash light for only a short period of time during which a shutter slit cannot move across the full width of the film, resulting in a partial or slit exposure of the film. This is likely to occur, for example, when the aformentioned two-curtain type shutter is operated at a high shutter speed, e.g. higher than 1/60 second, with the flash being fired. To avoid defective exposure with such two-curtain shutter slit mechanisms, camera operators have been advised to select a low or slow shutter speed for flash photography.
In another prior art camera flash device, as disclosed in Japanese published patent application No. Tokkosho 48-40421, the electronic flash tube is controlled to emit light at a constant intensity for a duration of time covering the traveling of the shutter slit. This light emission resembles that of an FP flash bulb and may enable uniform exposure of the film with slit type exposure wherein the shutter slit sweeps or scans over the film plane.
In this prior art camera flash device, however, the flash light maintained at a constant intensity (hereinafter called flat flash light) is initiated in synchronization with the ordinary X-type synchro switch of the camera. With the ordinary X-type synchro switch of the camera functioning with the two-curtain type focal plane shutter, the synchro switch contact is closed when the leading shutter curtain completes its travel. Therefore, in the case of high speed shutter operation, the trailing curtain will have already traveled for a considerable distance when the synchro switch contact is closed. This causes a part of the film to have been covered by the trailing shutter curtain before the initiation of the flash light, resulting in uneven exposure of the film. With an FP type synchro switch, which is closed prior to the start of exposure, no uneven exposure occurs. However, in consideration of the rising light intensity characteristics of an FP bulb, the FP synchro switch contact is normally designed to close 15 to 20 ms before the leading shutter curtain starts to uncover the film. In contrast thereto, the intensity of a normal electronic flash tube rises so rapidly that it may reach a desired intensity immediately after closure of the synchro switch such that the use of a normal flash tube with an FP type synchro switch results in the wasteful emission of flash light for a period of 15 to 20 ms before shutter operation, without contributing light to the film exposure.